I've just started milling boards on my cnc, which is a 400x300 mm work area, which is much more area than needed. I also have a 3018 size and it is completely capable of milling circuit boards.
There are a lot of videos on the single sided and double sided milling. I went straight to double sided and am on version 4 of a simple bjt buffer circuit. Screwed up the schematic on the first try, machining settings on second try, resized pads on board and moved some traces then milled and still wasn't satisfied, so changing milling settings again.
The software process learning curve and learning the correct milling settings seem to be what is taking the longest, but I'm almost to a working board, including solder mask.